Beagle: FDT support in BSP as a GSoC project?

[Christian Mauderer]

On 12/08/2019 08:18, Chris Johns wrote:
> On 12/8/19 3:51 pm, Christian Mauderer wrote:
>>
>> On 12/08/2019 03:33, Chris Johns wrote:
>>> On 12/8/19 9:22 am, Joel Sherrill wrote:
>>>>
>>>>
>>>> On Sun, Aug 11, 2019, 5:47 PM Chris Johns <chrisj at rtems.org
>>>> <mailto:chrisj at rtems.org>> wrote:
>>>>
>>>>     On 12/8/19 3:28 am, Joel Sherrill wrote:
>>>>     > On Sun, Aug 11, 2019, 10:59 AM Christian Mauderer <list at c-mauderer.de
>>>>     <mailto:list at c-mauderer.de>
>>>>     > <mailto:list at c-mauderer.de <mailto:list at c-mauderer.de>>> wrote:
>>>>     >
>>>>     >     Hello,
>>>>     >
>>>>     >     while mentoring Vijays GSoC project this year I noted that some drivers
>>>>     >     in the Beagle BSPs have quite horrible hard coded values for things like
>>>>     >     pinmux initialization. Maybe it would be a nice GSoC project for next
>>>>     >     year to replace this stuff with a fdt based initialization. I would like
>>>>     >     to ask for feedback before creating a ticket for it because it would
>>>>     >     mean a quite big change for the BSP (maybe even the name - see below).
>>>>     >
>>>>     >     Basically such a project would include the following parts:
>>>>     >
>>>>     >     - Parse the pinmux settings from FDT and create a two part driver for a
>>>>     >     'pinctrl-single' compatible FDT entry. One part generic, one device
>>>>     >     specific (similar to FreeBSD or Linux).
>>>>     >
>>>>     >     - Remove pinmux initialization from all drivers.
>>>>     >
>>>>     >     - Initialize drivers based on the FDT (instead of functions like
>>>>     >     bbb_register_i2c_1(...))
>>>>     >
>>>>     >     - Taking a more detailed look at the FDT what else could be initialized
>>>>     >     from it (maybe clocks?)
>>>>     >
>>>>     >     It could be a quite nice project for a RTEMS beginner. Due to the
>>>>     >     distributed initialization a lot of drivers have to be touched (at least
>>>>     >     i2c, spi and pwm). So a potential student would get a nice overview over
>>>>     >     the parts.
>>>>     >
>>>>     >     Note that this would be a big change for the BSP. Currently the BSP can
>>>>     >     be used without an FDT (as far as I know). Only libbsd needs one. After
>>>>     >     that a FDT would be mandatory. Despite that, I think that it would be an
>>>>     >     improvement.
>>>>     >
>>>>     >     Maybe it would be possible to merge the four beagle* BSPs that we have
>>>>     >     into only one "beagle" or "am33xx" BSP with that change. That would
>>>>     >     allow to support new Beagle variants like the Pocket Beagle without much
>>>>     >     effort (most likely only a change in the FDT).
>>>>     >
>>>>     >     What do you think? Should I create a ticket for it?
>>>>     >
>>>>
>>>>     I love it. Yes please create a ticket.
>>
>> OK. I'll create one in the next days.
>>
>>>>
>>>>     > I think this is a good idea if we can still avoid bloating apps with all
>>>>     > drivers. Make sure it has the right tags and shows up on the project page.
>>>>
>>>>     The beagle has a lot of RAM. Is this as important for this BSP?
>>
>> Most likely that's true for a lot of other FDT based BSPs too. Most of
>> the time FDT is used together with Linux systems.
>>
>>>>
>>>> Not really but we don't want bad patterns starting. 
>>>>>
>>> How does a user then configure a build to get the minimal set for them?
>>>
>>> Without dynamically loading does mean we need a bunch of build options? Is
>>> building for dynamic loading something we should consider?
>>
>> I think there would be two possibilities:
>>
>> 1. Introduce a module system similar to FreeBSD / libbsd. You
>> explicitely have to create a module reference if you want a driver.
>>
>> 2. Do it the other way round: Take care that device drivers are only
>> referenced via one init function. If a user wants a small config, he can
>> overwrite the function in his application which removes the driver.
>>
>> I would prefer 2 because:
>>
>> - Most likely it's simpler for the average user to just have everything
>> available without a special configuration.
> 
> I agree.
> 
>> - It's simpler to implement.
>>
>> - A user that really needs the last few bytes on a system like Beagle is
>> unlikely.
>>
>> - If someone really needs the bytes, most likely he knew that when
>> creating the draft of the system. I would expect that this is a more
>> experienced user who either knows of the tricks or asks on the mailing list.
>>
> 
> This is reasonable.
> 
> Thanks
> Chris
> 

I created a ticket for it: https://devel.rtems.org/ticket/3784